sdl: Flush the joypad events.

Bastien found a fix to the issue
"The lag after 10-15 minutes issue appears to be a bug in the sdl2 input
driver in RetroArch. RetroArch never clears the SDL event queue. After a
while it is so large it takes a significant time to traverse."

Fixes https://github.com/libretro/RetroArch/issues/7868

Solution thanks to Bastien.

input/drivers_joypad/sdl_joypad.c

@@ -399,6 +399,8 @@ static void sdl_joypad_poll(void)
             break;
       }
    }
+
+   SDL_FlushEvents(SDL_JOYAXISMOTION, SDL_CONTROLLERDEVICEREMAPPED);
 #else
    SDL_JoystickUpdate();
 #endif